Understanding Energy Changes in Chemical Reactions: Thermochemistry Guide

1. The In’s and Out’s of Energy in Reactions Thermochemical Equations

2. The Chef’s Recipe for Successful Cooking • If the only product is Ca(OH)2 (s), calculate the heat released per mole. • If the only product is Ca(OH)2 (aq), calculate the heat released per mole.

3. Energetic Objectives • Determine the heat absorbed or released by a chemical reaction using calorimetry. • Classify reactions as being either endothermic or exothermic based on experimental data. • Write a thermochemical equation that correctly reflects the stoichiometry of the reaction. • Use thermochemical equations to calculate energy changes in chemical reactions.

4. Endothermic and Exothermic Reactions – Pre Lab Items • Complete pre-lab questions (if you didn’t the night before) • Write hypothesis that answers the lab question in a complete sentence. You may discuss this with your lab partner. • Get questions checked/stamped by instructor BEFORE going into the lab. • SAFETY FIRST!!!

5. Endothermic and Exothermic Reactions – Lab Instructions • Follow all safety rules and wear safety gear correctly at all times. • Follow the directions on the student handout carefully. • Dispose of all used chemicals according to the directions. You will find a waste container at the front of the lab. • During last 10 minutes of lab, you will clean up your lab area and put away all materials you used. DO NOT REMOVE GOGGLES OR APRON UNTIL EVERYTHING IS CLEAN AND PUT AWAY. • After you have finished the lab, you will sit down with your partner at your desk and complete the calculations for the lab.

6. Endothermic and Exothermic Reactions – Post Lab

7. THERMOCHEMISTRY • Thermochemistry – study of heat changes that accompany chemical reactions and phase changes • in an exothermic reaction or process, heat energy is a product or is released • in an endothermic reaction or process, heat energy is a reactant or is absorbed • involves heat change in the universe: • system – specific part of the universe that contains the reaction or process you wish to study • surroundings – everything in the universe other than the system • universe = system + surroundings

8. ENTHALPY • enthalpy – (H) heat content of a system at constant pressure, measure of chemical potential energy in the system • difficult if not impossible to know the actual value; can’t easily measure • can, however, measure changes in enthalpy during a reaction • enthalpy (heat) of reaction – (ΔHrxn) – change in enthalpy for a reaction: ΔHrxn = Hfinal - Hinitial or ΔHrxn = Hproducts - Hreactants • Enthalpy can be determined through experimentation or using standard heats of formation ()

9. EXOTHERMIC VS ENDOTHERMIC EXOTHERMIC ENDOTHERMIC for an endothermic reaction, Hreactants< Hproducts therefore ΔHrxn is positive • for an exothermic reaction, Hreactants > Hproducts therefore ΔHrxn is negative Remember that Q is the heat lost or gained. The enthalpy change (ΔH) us equal to Q at constant pressure. Assume that Q = ΔHrxn in all problems.

10. Thermochemical Equations • Thermochemical equation – balanced chemical equation that includes the physical states of all reactants and products and the energy change (ΔHrxn) Example: The combustion of propane is an exothermic reaction: ΔHrxn = -2220 kJ/mol Example: The decomposition of calcium carbonate is an endothermic reaction: ΔHrxn = +176 kJ/mol Where did this number come from???

11. The Chef’s Recipe for Successful Cooking – a Second Look • If the only product is Ca(OH)2 (s), calculate the heat released per mole. • If the only product is Ca(OH)2 (aq), calculate the heat released per mole.

12. Energy and Stoichiometry The energy term in a thermochemical equation is just like any other substance when making comparisons. Example: If 85.00 g of propane are burned in excess oxygen, how much energy is released? Ans: -4290 kJ

13. Energy and Stoichiometry Example – How much lime can be produced if 486.0 kJ of energy are applied to the reaction below? Ans: 154.9 g CaO

14. Now It’s Your Turn! • Work with your lab partner to complete the post-lab questions. • Write a complete conclusion for the lab. Write using correct grammar, spelling, and punctuation. Use complete sentences and complete thoughts. • When you finish the post-lab questions and the conclusion, you will begin to work on the Thermochemical Equations Problem Set. HW: Complete the problems in the Thermochemical Equations Problem Set

15. What’s Your Energy Level? On a clean sheet of paper, answer the following questions. Show all work and calculations for full credit. • In a coffee cup calorimeter, 50.0 mL of 1.0 M NaOH and 50.0 mL of 1.0 M HCl are mixed. Both solutions were originally at 23.5°C. After the reaction, the final temperature is 29.9 °C. Assuming that all the solutions have a density of 1.0 g/mL, and a specific heat capacity of 4.184 J/g°C, calculate the enthalpy change for the neutralization of HCl by NaOH. Assume no heat is lost to the surroundings or the calorimeter. • Based on your answer to #1, is the reaction endothermic or exothermic? Explain using two pieces of your data. • Methyl alcohol (CH3OH) is completely combusted. Water vapor is one of its products. Write the balanced thermochemical equation for this reaction showing the enthalpy of the reaction ( ΔHrxn = -1277 kJ/mol) as either a reactant or a product. • Calculate the enthalpy change for the following reaction if only 25.00 g of calcium carbonate were used.